A widely used method for affixing toner materials to a receiver sheet is by the application of high temperature and pressure in the fusing subsystem of a photocopying machine. A common configuration for a fusing subsystem is to place a pair of cylindrical rollers in contact. The roller that will contact the side of the receiver sheet carrying the unfixed or unfused toner is known as the fuser roller. The other roller is known as the pressure roller. The area of contact is known as the nip.
The toner receiver sheet containing the unfixed or unfused toner is passed through the nip. A soft coating on one or both of the rollers allows the nip to increase in size relative to the nip which would have been formed between two hard rollers and allows the nip to conform to the receiver sheet, improving the fusing quality. Typically, one or both of the rollers are heated, either through application of heat from the interior of the roller or through external heating. A load is applied to one or both rollers in order to generate the higher pressures that are necessary for good fixing or fusing of the toner to the receiver sheet.
The application of high temperature and pressure as the receiver sheet passes through the nip causes the toner material to flow to some degree, increasing its contact area with the receiver sheet. If the cohesive strength of the toner and the adhesion of the toner to the receiver sheet is greater than the adhesion strength of the toner to the fuser roller, complete fusing occurs. However, in certain cases, the cohesive strength of the toner or the adhesion strength of the toner to the receiver is less than that of the toner to the fuser roller. When this occurs, some toner will remain on the roller surface after the receiver sheet has passed through the nip, giving rise to a phenomenon known as offset. Offset can also occur on the pressure roller.
Offset is undesirable because it can result in transfer of the toner to non-image areas of succeeding copies and can lead to more rapid contamination of all machine parts in contact with the fusing rollers and to increased machine maintenance requirements. It can also lead to receiver (paper) jams as the toner-roller adhesion causes the receiver sheet to follow the surface of the roller rather than being released to the post-nip paper path.
It is common in some machines to apply release oil externally to the roller in the machine as it is being used. External application of a release agent carries certain disadvantages. The release agent can be spread to other parts of the machine, causing contamination. Further, streaks may appear in the image as a result of imperfect spreading of the release agent across the roller surface. Therefore, it is desirable to improve the release performance of the roller materials in order to be able to minimize the amount of release agent that must be applied to the roller.
Use of a soft fuser roller and a harder pressure roller have been shown to facilitate release of the toned receiver from the fuser roller. Low surface energy materials such as fluorine-containing coatings or silicone rubbers have been used as fuser roller coatings. An example of a low surface energy material is described in U.S. Pat. No. 3,731,358 which discloses the use of a poly(dimethylsiloxane) (hereinafter sometimes referred to as "PDMS") as a coating for a fuser roller. This patent also discloses coating the roller with silicone oil after it has been used for a period of time in order to restore release properties.
In European Patent Application 0 417 814 A, there is disclosed a fuser roller having a PDMS outer layer which has filler that contains absorbed silicone oil. It also discloses a fuser roller having a low viscosity PDMS oil incorporated therein in a comparative example. Poor offset performance was noted.
Addition of a low viscosity silicone oil into the roller material during formulation has also been suggested to improve the release properties of the roller in: K. Imai, N. Hasebe, T. Asmi, S. Soga, I. Fukushima, and Y. Asahina, U.S. Pat. No. 4,074,001, 2/78.
There continues to be a need for improved fuser and pressure rollers with improved fusing performance, e.g. improved paper release, toner-paper adhesion or reduced "crack width".